The present invention relates to a fire monitoring system for detecting the occurrence of a fire, more particularly to a fire monitoring system for detecting the occurrence of a fire by monitoring a rise in temperature within a space to be monitored.
There have so far been proposed and put into practical use several systems for monitoring the temperature of a broad space to be monitored. Among them, the monitoring system in which the temperature is detected from the propagation speed of ultrasonic waves has been attracting attention since it involves less equipment but is capable of covering relatively broad space to be monitored. Japanese Unexamined Published Utility Model Application No. 96388/76, for example, discloses a fire sensing device utilizing ultrasonic waves as shown in FIG. 1. In detail, there are disposed an ultrasonic wave generator 20 and an ultrasonic wave receiver 21 arranged so as to confront each other at a suitable interval, and a light emitting source 22 and a light-receiving unit 23 arranged so as to confront each other and juxtaposed with the line joining the above two units. An intermittent oscillator 24 drives the ultrasonic wave generator to intermittently emit ultrasonic waves therefrom and at the same time causes the light emitting source 22 to emit light therefrom. A light signal received by the light-receiving element 23 triggers a gate signal generator 25 to keep open a gate circuit 26 disposed between the ultrasonic receiver 21 and a switching circuit 27 for a prescribed period of time. The period for keeping the circuit open is set to be shorter than the period, when the space to be monitored is at room temperature, from the time of driving the ultrasonic generator 20 to the time of arrival of an output signal of the ultrasonic receiver to the switching circuit 27. In other words, when the temperature of the space to be monitored goes up due to occurrence of a fire, the propagation speed of the ultrasonic waves is increased so that an output signal of the ultrasonic wave receiver 21 reaches the switching circuit 27 within the above mentioned prescribed period, and causes the relay switch 28 to be actuated.
With such a system for monitoring a temperature rise in the space to be monitored by use of ultrasonic waves, it is possible to linearly monitor the temperature and monitor a broad space by use of a single unit of the device.
However, when the space to be monitored is too broad, there may be cases where a local rise in temperature is difficult to be accurately detected. For example, when a monitoring space extending linearly from the ultrasonic wave generator to the ultrasonic wave receiver is assumed to comprise an assembly of imaginary subspaces, only accumulated value of the propagation time for traversing each subspace is obtained as a result of the detection. Therefore, since there may be some subspaces in which temperature rise is taking place locally and also there may be some subspaces in which temperature fall is taking place locally, it may occur that the accumulated value is approximately equal to zero, and therefore it is not possible to detect the temperature rise.
Further, the propagation speed of ultrasonic waves in the air varies with the humidity as shown in the following table.
______________________________________ Speed of Sound Temperature Humidity 0% Humidity 100% ______________________________________ 20.degree. C. 343 (m/s) 345 (m/s) 40 355 359.5 60 366 377 80 377 404 ______________________________________
As can be seen from the table, the influence of the humidity on the propagation speed of the ultrasonic waves will become conspicuous at temperatures above 60.degree. C., although it is of little significance at room temperature. In particular, in the temperature range of 60.degree.-80.degree. C. where occurrence of a fire is to be judged, the influence of the humidity can no longer be disregarded. Moreover, it is well known the humidity goes up due to the burning of lumber and the like, so that the rise in the humidity cannot be neglected in judging the occurrence of a fire.